Optical encoders or tachometers are well known in the art. Basically, these devices comprise a stationary light source arranged to be mounted on a motor close to the motor shaft, a stationary receiving photo electric cell in alignment with the light source and a rotating shutter arranged to be secured to the motor shaft to rotate between the light source and photo cell and thus optically generate a series of pulses of frequency corresponding to the R.P.M. of the motor. In these assemblies it is vitally important that the light source and photo cell be properly aligned with respect to the rotating shutter in order that usable output pulses are properly generated. Any misalignment could result in the absence of pulses or very weak pulses if the full apertures defined by the rotating shutter are not utilized.
There is generally no problem when an optical encoder is installed on a motor in the factory. Proper alignment jigs and the like are readily available and once the selection is complete on the motor, there is ordinarily no problem of misalignment unless an ultimate user attempts to disassemble the optical encoder from the motor and attempts to reassemble the same himself.
On the other hand, it is presently common practice to provide optical encoder kits which may be shipped directly to a user and assembled by the user himself on a motor. Normally, the basic component of the assembly constitutes a mounting member carrying the light source. This member is provided with a central opening normally of a size greater than the diameter of the motor shaft of any particular motor with which the encoder is to be used. The larger sized opening is necessary to provide a definite clearance between the rotating shaft and the mounting member itself carrying the light source which latter member is fixed to the motor casing in a position surrounding the shaft and is stationary. The next component of the assembly constitutes the shutter member which is mounted directly to the motor shaft. Finally, an appropriate top cover carrying the photo cell detector overlies the shutter member and is affixed to the mounting member so that the shutter member is sandwiched therebetween and will rotate between the light source and photo cell.
Because of the larger sized central opening of the mounting member relative to the motor shaft, in affixing the mounting member to the motor casing in a stationary position, it is essential that it be exactly coaxially centered relative to the motor shaft. That is, the outer circular periphery of the mounting member must be exactly concentric with the axis of the motor shaft. Otherwise, the rotating shutter which is affixed to the shaft will be out of alignment with the light passed from the light source to the photo cell all as described heretofore.
In view of the foregoing, it is customary to provide a centering means such as a bar with a V-groove affixed to the mounting member at the factory to permit exact centering of the mounting member prior to securing it to the motor. After securement of the mounting member, this centering bar or fixture can be disconnected. However, if it is ever desired to remove the optical encoder and place it on another motor or even reassemble it on the same motor, the mounting member must be sent back to the factory for proper securement of the bar fixture thereto in order that proper centering can again be achieved.
The foregoing problem has been solved in the provision of optical encoders in kit form in accord with the teachings of my prior U.S. Pat. No. 4,102,028 issued July 25, 1978. In my aforesaid patent, I disclose a centering fixture which can be provided with the kit and which functions to enable precise centering of the mounting member to a motor casing in order to solve the above-noted problems without having to send the assembly back to the factory.
However, effective operation of my centering fixture as described in the above noted patent depends on a precise central bore opening in the mounting member of a specific diameter. If the diameter of this central opening in the mounting member should be different, then a different centering fixture must be provided. Moreover, should there be any minor variations in the size of the central opening of the mounting fixture, the centering fixture as described in my above patent will not function properly.